Deformational Characteristics And Geochronological Constraints On The Karasu Ductile Shear Zone,Southern Pamir

Generally,the Pamir western syntaxis of the Tibetan plateau has been divided into four tectonic regions: the Northern Pamir,Central Pamir,Southern Pamir,and Karakoram terranes.It has experienced a long complicated processes of the Paleozoic–Mesozoic terranes collision and then overprinted by the Cenozoic India-Asian collision,created a complex structural pattern.This area also has played an important role in the tectonic evolution of the Tibetan Plateau.However,study of Mesozoic tectonic evolution in this area remains very insufficient.In this thesis,the structural geometries,kinematics,geochemical features,and forming time of the Southern Pamir Karasu ductile shear zone have been investigated based on the field deformation characteristics,microstructural characteristics,geochemical data analysis,and zircon U-Pb LA-ICP-MS dating.Field observations show that the ductile shear zone is NW–SE-striking,with a width of about 1.5km and a length of more than 10 km.The rocks within the ductile shear zone consists of three lithologic sections as follows: protomylonitic marble,granitic mylonite and blastomylonite,respectively.The ductile shear exhibits a gradual weakening tendency from the center to the edges.The mylonitic foliation of the ductile shear zone is steep and the stretching lineation is developed extensively.According to the grain-tail complexes,asymmetric fold,domino structure,S-C fabrics,sheath folds and other shear indicators observed in the field and under the microscope,the ductile shear zone is a sinistral strike-slip ductile shear zone and with partial thrust component(transpression).From formation of the ductile shear to being uplifted to the surface,this shear zone has undergone a tectonic evolution from ductile deformation,to brittle-ductile,ductile-brittle,and then brittle deformation.According to the deformation characteristics of minerals in different parts of the shear zone,the deformation temperature of the edge of the ductile shear zone is estimated to be around 200-250 °C,and the deformation temperature of the syn-tectonic granite rock can reach to 400-500 °C.The deformation temperature in extensive deformation areas can reach up to 500-600 °C.Moreover,the granite in the shear zone is suggest to a syn-tectonic granite according to the morphology,deformation,geochemical characteristics,a comparison with other plutons in this region,and combined with the predecessor's discriminative signs of the pre-tectonic,syn-tectonic and post-tectonic granites within the ductile shear zone.Two syn-tectonic granitic samples yield the U-Pb ages of 233.2±1.8Ma and 235.7±1.4Ma,respectively,which indicate that Karasu ductile shear zone was formed in the earliest of Late Triassic.According to the tectonic evolution model of the Pamir region,Rushan Ocean is subducted beneath the Southern Pamir during the Late Triassic,and the formation of the Karasu ductile shear zone indicates that the interior of the Southern Pamir has undergone significant tectonic deformation in the early of Late Triassic.